The exploitation of biological macromolecules, such as nucleic acids, for the fabrication of advanced materials is a promising area of research. Although a greater variety of structural and functional uses can be envisioned for protein-based materials, systematic approaches for their construction have yet to emerge. Consistent with theoretical models of polymer macrocyclization, we have demonstrated that, in the presence of dimeric methotrexate (bisMTX), wild-type Escherichia coli dihydrofolate reductase (DHFR) molecules tethered together by a flexible peptide linker (ecDHFR(2)) are capable of spontaneously forming highly stable cyclic structures with diameters ranging from 8 to 20 nm. The nanoring size is dependent on the length and composition of the peptide linker, on the affinity and conformational state of the dimerizer, and on induced protein-protein interactions. Delineation of these and other rules for the control of protein oligomer assembly by chemical induction provides an avenue to the future design of protein-based materials and nanostructures.

Chemically Controlled Self-Assembly of Protein Nanorings

The aggregates in heated bovine β-lactoglobulin solutions were separated by high-performance size-exclusion chromatography, and molecular characteristics were measured on-line using multiangle laser-light scattering detection (SEC-MALLS). This technique was proven to be a very useful and valuable approach for characterization of heat-induced β-lactoglobulin aggregates. Using TSK G2000 SW XL and TSK G4000 SW XL silica gel columns connected in series, aggregates up to a molecular mass of 4 x 10 6 Da could be separated, and complete molecular mass distributions were derived. Up to a molecular mass of 1.5 x 10 6 Da the molecular masses calculated by the MALLS agreed very well with the results obtained by conventional calibration. The measured molecular mass distributions of the heat-induced β-lactoglobulin aggregates varied strongly with experimental heating conditions (β-lactoglobulin concentration, pH, heating temperature, ionic strength). The results obtained with several β-lactoglobulin concentrations at neutral pH were consistent with a kinetic aggregation model based on thiol/disulfide exchange reactions. The average molecular mass and the radius of gyration of the heat-induced β-lactoglobulin aggregates increased with increasing initial β-lactoglobulin concentration.

Molecular Mass Distributions of Heat-Induced β-Lactoglobulin Aggregates

Aus dem sterischen Verlauf und aus der Geschwindigkeit der thermischen Isomerisierung von Cyclobutenen zu Butadienen folgt ein grundsatzlicher Unterschied im Mechanismus fur monocyclische Cyclobutene und fur Cyclobutene, die in 3.4-Stellung mit 4-, 5- oder 6-gliedrigen Ringen kondensiert sind. Nur im ersten Fall leistet die Doppelbindung Hilfestellung fur die Ringoffnung, wahrend im zweiten Fall die Ringspannung als einzige treibende Kraft wirkt.

Valenzisomerisierungen von Cyclobutenen

Four-membered rings can be cleaved thermally, Photochemically, or catalytically into two π bonded fragments. Theoretical calculations, kinetic studies, and investigations of stereo- and regioselectivity have been undertaken to clarify the question of whether the reaction involves one or two steps and to permit predictions on its course. [2 + 2]-Cycloreversions have been used to clarify the structure of four-membered rings, to prepare highly reactive π electron-systems and–in combination with a [2 + 2]-cycloaddition–to protect double bonds. The combination of a cycloaddition and-reversion can be used to convert a carbonyl group into an olefin. Starting with compounds containing annelated four-membered rings, compounds with two functional groups or large ring systems can be prepared. [2 + 2]-Cycloreversions have also been discussed in connection with storage of solar energy.

[2 + 2]‐Cycloreversions

β-Lactoglobulin, a protein belonging to the lipocaline family, is studied by Photon correlation spectroscopy and small angle X-ray scattering in acidic solutions (pH = 2.3) and at ionic strengths in the range 7−507 mM. Both experiments give a clear evidence of a protein monomer−dimer equilibrium affected by the ionic strength of the solution. The interparticle interactions are determined by means of Photon correlation spectroscopy by following the trend of the protein diffusion coefficient versus concentration (from 0.2 to 25 g/L) at ionic strengths of 7 and 107 mM. From a detailed analysis, estimates of the protein charge, hydrodynamic radius, and dimeritazion fraction have been obtained. Small angle X-ray scattering measurements performed versus ionic strength (from 7 to 507 mM) in dilute solutions (10 g/L) allow an independent estimate of the dimerization fraction. A global fit of the SAXS scattered intensities at different salt concentrations using a simple electrostatic model for the aggregation mech...

Maria. Zupan

Papers

The Kinetics of the Thermal Decomposition of Methyl Cyclobutanecarboxylate